The object of the invention is a method of providing by means of a laser beam on a bright metal surface areas discernible from the surrounding surface as darker areas, for producing for instance optically readable marks and a metal surface comprising areas coloured in this way, the surface being substantially smooth within the coloured areas and the coloured areas being mainly at the same level as the metal surface surrounding these areas.
Lasers are commonly used in the packaging industry for marking dates, symbols and bar codes on packages. Further, lasers are used for marking various machine parts. The used marking methods may be roughly divided into two groups depending on whether the mark is produced
a) by engraving on surface (melting, vaporization)
b) by providing a chemical reaction/a change in microstructure by means of a laser beam (heating).
Generally, Nd-YAG and CO.sub.2 lasers are used for marking, the effect of laser pulses being based on the heat effect of the beam (wavelengths of the pulses are within the infrared area).
A pattern to be marked is formed either by a mask, through which a laser beam passes, or by moving a focused laser beam/body to produce the desired pattern.
Engraving on surface (e.g. metals, plastics) means that the surface layer of a body is melted or material is vaporized from that layer. A mark discernible from the surface is thus produced. Such methods appear for instance from published applications EP 0 134 469 and WO 88/10475. The contrast created may be improved by various methods, as described for instance in published application WO 89/07302.
One alternative to surface engraving is to coat the surface to be marked with a material creating a good contrast with the basic material. By removing the coating by a laser, a mark having a good contrast is produced. A procedure like this is disclosed in U.S. Pat. No. 4,707,722, for instance. The same method is used e.g. for marking soft drink bottles, in which method the printing ink of a label is removed by a laser from the surface of a backing paper. A light backing paper and the printing ink create a good contrast, due to which the mark is easily readable.
Marks produced by the above marking methods are characterized in that they break the original surface and are situated at a level lower than the original surface.
A chemical reaction may also be utilized for marking. There are various organic coating agents (lacquer), which turn dark under the influence of a laser beam (under the influence of heat or UV radiation introduced) within an exposed area. In the same way, the metal may at first be anodized (an oxide layer on titanium, for instance) and then the oxide produced may be reduced by means of a laser, whereby a discernible contrast is created between the oxide and the reduced area. In this manner, a readable trace can be produced. Such a procedure appears e.g. from U.S. Pat. No. 4,547,649.